Bumps and ripples in the otherwise flat ring system of Saturn cast long shadows at equinox. Image credit: NASA/CassiniImagine a vast, flat plain spreading out before you for tens of thousands of miles in all directions, with no Earthly curvature to give the horizon its slightly finite look. Instead, it stretches seemingly to the infinite blackness of space in one direction, and slices straight into the streaky, wind-smoothed clouds of Saturn in the other…

Hard to imagine what it would be like to float just above the rings of Saturn, but what a sight it must be! As a kid, one of my favorite astronomical pass-times was imagining the view from other places in the Solar System.

Now imagine a towering bulge of frosty mist rising up out of this super-flat plane of ice chunks, literally the size of a mountain. Such is what was beheld by NASA's Cassini spacecraft last month–albeit, from a distance–when it turned its cameras to Saturn's vast rings during the few days surrounding Saturn's equinox (August 29, 2009), giving us a view never before seen.

Equinox on Earth, when the Sun is positioned directly over our equator, happens twice a year. Due to Earth's tilted rotational axis, as we orbit the Sun the latitude over which the Sun shines directly cycles north and south between the latitudes of the Tropics. On its way north to warm our (Northern Hemisphere) summers or south to leave us in the chill, the Sun crosses the equator on the equinoxes (Fall and Spring).

The same thing happens on Saturn, with two differences. First, Saturn takes nearly 30 years to orbit the Sun, so equinox comes only about every 14 years. Second, Saturn has its system of rings that encircle the planet directly above its equator, serving as a visible extension of the equator. At Saturn's equinox, the Sun is not only directly over the equator, but sunlight strikes the rings edge-on, like a flashlight shining on a flat piece of paper from the edge, the light just grazing over the surfaces on either side.

When this happens, any deviations from the flatness of the ring system—bumps and ripples–cast long shadows across the rings, making the features much easier to see. The same thing is seen on that piece of paper with shadows from creases and bumps leaping across the page.

As seen from Earth, equinox on Saturn means the rings appear to vanish as we look at them edge-on. This behavior puzzled astronomers long ago before they understood the rings for what they are. During the August 2009 Saturn equinox, however, for the first time in history we had a bird's-eye view of the rings during equinox, from Cassini. Cassini has been in orbit around Saturn for five years now.

Cassini spotted a number of prominent shadows trailing bright spots and ridges—bumps and ripples of different sorts rising above the ring plane.

Some of the bumps–icy ring material kicked up by the gravitational disturbance of a small moonlet inside the rings–were measured at over two miles high, the height of the Rocky Mountains. Other rippling features, such as long ridges running along the direction the rings encircle Saturn, are waves created by the gravity of moons orbiting outside the ring system. Still other types of disturbances observed are possibly caused by the impact of meteoroids or chunks of ice with the rings.

Saturn's rings are tens of thousands of miles across, but are extremely thin—perhaps no thicker than the height of a four-story building! So a bump or ripple as high as a mountain is a big deal!

Ah, to be on Saturn, now that equinox is here…

NASA's MESSENGER spacecraft at Mercury-artist concept.

Photo by: NASA

I've been waiting for the "whole story" on Martian ice at the Phoenix lander site to unfold more completely, but the chemical analyses have not yet run their full courses-so I've decided to widen the focus on this blog to give a status report on current active robotic exploration of planets going on around the Solar System.

Limiting my scope to only planetary spacecraft, the list is still respectable. In no particular order, here's the round-up:

Spirit: Mars Exploration Rover Spirit's activities on the Martian surface have been reduced to save on power, but the robot remains alive. With the arrival of Martian winter, Spirit spends more power running heaters to keep key electronic and power equipment healthy. Spirit remains in the giant Gusev Crater, where it will spend its entire life on Mars.

Opportunity: Exploring a much smaller crater of its own, Victoria Crater-Spirit's twin, Opportunity, continues its investigation of the rock layers of Mars' geological history. As of June 10, Opportunity has clocked in at 7.26 miles of total "roving" on Mars, since its landing back in 2004.

Phoenix: The brand-spankin'-new Mars Phoenix lander has been digging into one of Mars' greatest scientific mysteries: water. Detailed chemical analysis of samples taken at Phoenix's site near the northern polar ice cap is underway, but the big question– is Phoenix standing on frozen Martian water– has been answered: yes.

Mars Reconnaissance Orbiter: The newest orbiter in the Martian fleet continues to send back its extreme-high-resolution imagery and its revealing chemical measurements, as well as to serve as a high-speed data and communication relay for other Mars-exploring robots.

Mars 2001 Odyssey: Credited with detecting the massive amounts of frozen water in Mars' northern hemisphere-the same ice that the Phoenix lander is now scraping at, Mars 2001 Odyssey continues its surveillance of Mars' chemistry and atmosphere.

Mars Express: The European orbiter that launched the ill-fated Beagle II lander has continued on a respectable career of exploration in its own right. Mars Express also helped support the landing of the Phoenix.

Cassini: Saturn's first robot-in-residence, Cassini, has concluded its initial 4-year mission and is now continuing on an extended mission. Cassini has given us unprecedented close-up images and measurements of many of Saturn's stunning moons, its complicated ring system, and the swirling, aurora-touched cloud formations of Saturn itself.

MESSENGER: The first spacecraft to visit the little-understood Mercury since 1975 made its first flyby of that planet last January, and will settle into a permanent orbit in March 2011. Even the few pics it snapped as it hurled by gave us far more detailed images of Mercury than ever before.

New Horizons: Launched a couple years ago on its outward bound, meteoric flight to Pluto, New Horizons has already performed some exploration duty, capturing images and data of Jupiter, Jupiter's volcanic moon Io, and Jupiter's long magnetic "tail." Now in "cruise mode," this little robot will fly past Pluto (dwarf planet; king of the Plutoids) in July 2015.

Voyagers 1 and 2: Do you remember the remarkable voyages of discovery made by the Voyager spacecraft, both launched in 1977? Since completing their primary missions of flying by the Gas Giant planets (Voyager 1 at Jupiter and Saturn, Voyager 2 at all four), these two veterans have continued to operate and send information back to Earth, and are now about 3 times more distant from the Sun than Pluto.

That's the wrap. If I missed anyone, my apologies!

Artist concept of a geyser erupting on Enceladus.
Credit: David Seal.Back when I was young…okay, a previous generation might have ended that sentence with, "…I’d walk forty miles through the snow to get to school…" But I'm not exaggerating when I say, when I was young we knew next to nothing about faraway places in the Solar System…such as the moons of Saturn.

A layer of the veil around Saturn’s moons was removed when Pioneer 11 and Voyagers 1 and 2 made flybys of Saturn in the '70s and '80s. The Saturnian moons, it appeared, were not the lumps of rock and dust that Earth's own Moon is made of, but objects containing no small amount of water ice. Not terribly surprising, considering the low temperatures of the outer solar system where ice-rich comets roam.

Visions of frozen alien landscapes, replete with icicles and ice cliffs and ice fields and ice ice ice! were conjured in my imagination, and in artist depictions of majestic ringed Saturn seen from moons like Rhea or Dione or Enceladus.

Four years ago, Saturn’s first permanent visitor from Earth–the Cassini spacecraft–arrived there, and since has been making extreme closeup examinations of Saturn, its rings, and its increasingly wondrous and beautiful moons. Cassini is almost literally ripping apart veil after veil of our ignorance of these little worlds.

Far from a contingent of enormous but simple snow cone balls, Cassini has shown us that some of Saturn’s moons are apparently alive with liquid motion. First, there were the surface “lakes” and “seas” on Titan, probably made of extremely cold liquid hydrocarbons like methane and ethane–the stuff that spouts out of the gas range in your kitchen. Lakes and seas and rolling waves of liquid natural gas are fine and dandy for an imagined shoreline scene–but take a dip in those "waters" and an actual water-based creature like you would freeze solid in seconds. Scenic, but not inviting for a swim…

But recent observations by Cassini have shown that Titan's frigid unearthly lakes and Enceladus' snowball exterior may just be additional veils that are now being lifted.

In March, Cassini flew within 30 miles of the surface of Enceladus and right through a plume of material venting into space from the moon’s interior—an enormous "geyser." Earlier observations had sensed the presence of water in the plume, giving rise to speculation that liquid water in some form might exist beneath Enceladus' surface—perhaps chambers of liquid heated by tidal stressing of the interior.

When Cassini flew through the plume, its chemical sensors "sniffed" more than just water in the stream, but a good deal of organic molecules as well…not unlike material found in comets, stuff left over from the formation of the Solar System that may have been the building blocks of life on Earth.

The other "water find" was that of a possible liquid ocean under the crust of Titan–similar perhaps to the deep liquid water ocean believed to exist under the surface of Jupiter's moon Europa. Unexpected "drift" in the locations of landmarks on Titan's surface is what suggests a liquid ocean–water with perhaps some ammonia–that the frozen crust may be floating on.

With all the liquid water and organic chemistry being revealed in the Saturn system (and elsewhere in the outer solar system), our imaginations can shift from the older standards of envisioning otherworldly landscapes of sculpted ice or even seascapes of liquid hydrocarbon lapping on shores of water ice sand, to something a little more, shall we say, "lively…"?

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.